Assignment In Conditional

csharp.assignment_in_conditional

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

CSharp

Description

Reports a simple assignment (x = expr) appearing in the condition expression of an if, while, do … while, or for statement. A simple assignment in a boolean context is almost always a typo for ==. In C# the typo only compiles when the assigned value is itself bool, which makes the mistake easy to overlook in code review.

Rationale

if (x = something) evaluates something and assigns it to x, then uses the assigned value as the condition. The intended code is almost always if (x == something). The rule excludes compound assignments (+=, -=, ??=, …) because they occasionally appear legitimately inside loop headers.

if (ok = TryParse(s))             // FLAW — simple assignment in condition
{
    Use(s);
}

while (running = Poll())          // FLAW — assignment, not comparison
{
    Step();
}

if (ok == TryParse(s))            // OK — proper comparison
{
    Use(s);
}

Remediation

If a comparison was meant, change = to ==. If the assignment was intentional, lift it out of the condition into a preceding statement, or wrap it in extra parentheses to make the intent obvious.

References

C# assignment operator